(1) Field of the Invention
The invention relates generally to wheelchairs, and more particularly to lever-propelled wheelchairs providing variable mechanical advantage for various terrain and use conditions.
(2) Description of the Prior Art
In the prior art, the usual situation respecting the user of a wheelchair is recognized, i.e. that the user has little or no leg strength, but does have arm and upper torso strength such that propulsion can be effected through operation of hand levers. Moreover, the prior art discloses various gear-shifting arrangements to accommodate various terrain, slopes and other conditions.
Bicycles with variable drive ratios have long been known, mostly with chain drive and multiple sprocket wheels among which the chain is positioned for drive ratio changing. A few variations for bicycle drive ratio control using gear drives or combinations of gear and chain drives are known. U.S. Pat. Nos. 3,863,503 and 614,969 show forms of the first named variation and U.S Pat. No. 3,948,542 describe the second. Of course, bicycles are always foot pedal driven devices by their nature and, therefore, their various structural arrangements are quite different, vis-a-vis the arrangement of the invention herein to be described and not adaptable thereto.
The prior art terms "velocimon" and "velocipede" have been used to describe the types of user powered vehicle which generally includes the various wheelchair arrangements extant. The user powered wheelchair as a specialized vehicle of the class usually includes hand levers pumped forward and backward or a handwheel laterally outward from, but generally concentric with, the main or groundwheels of a wheelchair.
The variations extant include shiftable gear drives between the activating levers or hand wheels and the driver axles of the main (ground) wheels and chair and sprocket or belt and fully torque/speed modifiers. U.S. Pat. Nos. 3,917,311 and 605,530 are typical of invalid vehicles of those types.
In U.S. Pat. No. 390,174 and published European Patent application publication No. 4,205 (filed Mar. 14, 1979), the concept of coupling propulsion force into a ratchet (toothed) wheel from hand pumped levers by means of pawls associated with the levers is disclosed. The ratchet wheels connect to gear assemblies in the aforementioned U.S. Pat. No. 390,174 and more or less directly to the axles of the main wheels in the device of the European patent application, but no "gear-shifting" capability is included.
Any arrangement of gears inherently introduces friction losses which reduce the efficiency of the overall transmission. Moreover, gear arrangements are relatively expensive to manufacture to close tolerance as needed to minimize such friction losses. Still further, incorporation of gear transmissions increases the weight of the vehicle and, therefore, the difficulty of propelling it.
The manner in which the invention eliminates the prior art disadvantages and advances the state of the art will be understood as this specification proceeds.